For me, a good sex is a passionate sex. When you feel that the more you give, the closer you are to another person, and that person wants to give more to you too. It's sometimes hard to be close with someone emotionally (INFJ thing I guess), but when you're intimate with someone, and they feel the same for you as you do for them, you can get super close - both physically and intimately. It's like when you build a relationship, you're building a bridge. When you have great sex together, it's like dancing on that bridge.
What I also found out about myself and that kind of relates to it too, is that if I start dating someone and we have sex soon into the relationship, I immediately withdraw afterwards, even if the sex was good. It's like a defence mechanism - I let you in too far, too son; because if you slept with me, you know how passionate and intimate I really can be - and it's to early for you to know that.